Hybrid zones: Windows on climate change
Defining the impacts of anthropogenic climate change on biodiversity and species distributions is currently a high priority. Niche models focus on predicted changes in abiotic factors and generally ignore how species interactions and adaptive evolution impact the ability of species to persist in the face of changing climate. In continentally distributed species, population density often declines at range margins, making range boundaries diffuse and difficult to map. We suggest that long-term monitoring of hybrid zones will provide data on distributional changes and altered species interactions and can also document adaptive introgression of climate-relevant phenotypes or genotypes, if it occurs. As a test of our hypothesis we examined the dynamics of the zone of contact between Black-capped and Carolina Chickadees along a long-term transect in southeastern Pennsylvania using genomic, citizen science, and climate data.
We provide strong evidence that cold winter temperatures limit the northern extent of the Carolina Chickadee’s range and, using genomic data, confirm the rapid northward movement of this zone of contact over the past decade. For nearly half a century biologists have been using morphological and molecular markers to characterize species interactions within hybrid zones. During that time, the pace of anthropogenic climate change has accelerated and species have responded in turn. Range expansions, contractions, and species extinctions are becoming commonplace, but we are not always well prepared to document these changes, or predict them. As we demonstrate with the chickadee hybrid zone we are now poised to utilize both historical and current sampling of hybrid zones to understand how species interactions and adaptation are being transformed by our changing climate. We emphasize that hybrid zones should be sampled with this change in mind. We also highlight the utility of new genomic methods (increased genomic and geographic representation, use of historical DNA samples) and public databases (citizen science) that will facilitate studies of hybrid zone movement. As the approaches we suggest are adopted, hybrid zones will continue to act as windows on evolution and ecology in our changing world.